Assert | Node.js v26.5.0 Documentation

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- Assert
  - Strict assertion mode
    - Message parameter semantics
  - Legacy assertion mode
  - Class: `assert.AssertionError`
    - `new assert.AssertionError(options)`
  - Class: `assert.Assert`
    - `new assert.Assert([options])`
  - `assert(value[, message])`
  - `assert.deepEqual(actual, expected[, message])`
    - Comparison details
  - `assert.deepStrictEqual(actual, expected[, message])`
    - Comparison details
  - `assert.doesNotMatch(string, regexp[, message])`
  - `assert.doesNotReject(asyncFn[, error][, message])`
  - `assert.doesNotThrow(fn[, error][, message])`
  - `assert.equal(actual, expected[, message])`
  - `assert.fail([message])`
  - `assert.ifError(value)`
  - `assert.match(string, regexp[, message])`
  - `assert.notDeepEqual(actual, expected[, message])`
  - `assert.notDeepStrictEqual(actual, expected[, message])`
  - `assert.notEqual(actual, expected[, message])`
  - `assert.notStrictEqual(actual, expected[, message])`
  - `assert.ok(value[, message])`
  - `assert.rejects(asyncFn[, error][, message])`
  - `assert.strictEqual(actual, expected[, message])`
  - `assert.throws(fn[, error][, message])`
  - `assert.partialDeepStrictEqual(actual, expected[, message])`
    - Comparison details

## Assert#

Source Code: lib/assert.js

Stability: 2 - Stable

The `node:assert` module provides a set of assertion functions for verifying invariants.

### Strict assertion mode#

Added in: v9.9.0
History

Version | Changes
--- | ---
v15.0.0 | Exposed as `require('node:assert/strict')`.
v13.9.0, v12.16.2 | Changed "strict mode" to "strict assertion mode" and "legacy mode" to "legacy assertion mode" to avoid confusion with the more usual meaning of "strict mode".
v9.9.0 | Added error diffs to the strict assertion mode.
v9.9.0 | Added strict assertion mode to the assert module.

In strict assertion mode, non-strict methods behave like their corresponding strict methods. For example, `assert.deepEqual()` will behave like `assert.deepStrictEqual()`.

In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.

#### Message parameter semantics#

For assertion methods that accept an optional `message` parameter, the message may be provided in one of the following forms:

- string: Used as-is. If additional arguments are supplied after the `message` string, they are treated as printf-like substitutions (see `util.format()`).
- Error: If an `Error` instance is provided as `message`, that error is thrown directly instead of an `AssertionError`.
- function: A function of the form `(actual, expected) => string`. It is called only when the assertion fails and should return a string to be used as the error message. Non-string return values are ignored and the default message is used instead.

If additional arguments are passed along with an `Error` or a function as `message`, the call is rejected with `ERR_AMBIGUOUS_ARGUMENT`.

If the first item is neither a string, `Error`, nor function, `ERR_INVALID_ARG_TYPE` is thrown.

To use strict assertion mode:

    import { strict as assert } from 'node:assert';
    const assert = require('node:assert').strict;
    javascript

    import assert from 'node:assert/strict';
    const assert = require('node:assert/strict');
    javascript

Example error diff:

    import { strict as assert } from 'node:assert';

    assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected ... Lines skipped
    //
    //   [
    //     [
    // ...
    //       2,
    // +     3
    // -     '3'
    //     ],
    // ...
    //     5
    //   ]
    const assert = require('node:assert/strict');

    assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected ... Lines skipped
    //
    //   [
    //     [
    // ...
    //       2,
    // +     3
    // -     '3'
    //     ],
    // ...
    //     5
    //   ]
    javascript

To deactivate the colors, use the `NO_COLOR` or `NODE_DISABLE_COLORS` environment variables. This will also deactivate the colors in the REPL. For more on color support in terminal environments, read the tty `getColorDepth()` documentation.

### Legacy assertion mode#

Legacy assertion mode uses the `==` operator in:

- `assert.deepEqual()`
- `assert.equal()`
- `assert.notDeepEqual()`
- `assert.notEqual()`

To use legacy assertion mode:

    import assert from 'node:assert';
    const assert = require('node:assert');
    javascript

Legacy assertion mode may have surprising results, especially when using `assert.deepEqual()`:

    // WARNING: This does not throw an AssertionError in legacy assertion mode!
    assert.deepEqual(/a/gi, new Date());
    cjs

### Class: `assert.AssertionError`#

- Extends: `<errors.Error>`

Indicates the failure of an assertion. All errors thrown by the `node:assert` module will be instances of the `AssertionError` class.

#### `new assert.AssertionError(options)`#

Added in: v0.1.21

- `options` `<Object>`
  - `message` `<string>` If provided, the error message is set to this value.
  - `actual` `<any>` The `actual` property on the error instance.
  - `expected` `<any>` The `expected` property on the error instance.
  - `operator` `<string>` The `operator` property on the error instance.
  - `stackStartFn` `<Function>` If provided, the generated stack trace omits frames before this function.
  - `diff` `<string>` If set to `'full'`, shows the full diff in assertion errors. Defaults to `'simple'`. Accepted values: `'simple'`, `'full'`.

A subclass of `<Error>` that indicates the failure of an assertion.

All instances contain the built-in `Error` properties (`message` and `name`) and:

- `actual` `<any>` Set to the `actual` argument for methods such as `assert.strictEqual()`.
- `expected` `<any>` Set to the `expected` value for methods such as `assert.strictEqual()`.
- `generatedMessage` `<boolean>` Indicates if the message was auto-generated (`true`) or not.
- `code` `<string>` Value is always `ERR_ASSERTION` to show that the error is an assertion error.
- `operator` `<string>` Set to the passed in operator value.

    import assert from 'node:assert';

    // Generate an AssertionError to compare the error message later:
    const { message } = new assert.AssertionError({
      actual: 1,
      expected: 2,
      operator: 'strictEqual',
    });

    // Verify error output:
    try {
      assert.strictEqual(1, 2);
    } catch (err) {
      assert(err instanceof assert.AssertionError);
      assert.strictEqual(err.message, message);
      assert.strictEqual(err.name, 'AssertionError');
      assert.strictEqual(err.actual, 1);
      assert.strictEqual(err.expected, 2);
      assert.strictEqual(err.code, 'ERR_ASSERTION');
      assert.strictEqual(err.operator, 'strictEqual');
      assert.strictEqual(err.generatedMessage, true);
    }
    const assert = require('node:assert');

    // Generate an AssertionError to compare the error message later:
    const { message } = new assert.AssertionError({
      actual: 1,
      expected: 2,
      operator: 'strictEqual',
    });

    // Verify error output:
    try {
      assert.strictEqual(1, 2);
    } catch (err) {
      assert(err instanceof assert.AssertionError);
      assert.strictEqual(err.message, message);
      assert.strictEqual(err.name, 'AssertionError');
      assert.strictEqual(err.actual, 1);
      assert.strictEqual(err.expected, 2);
      assert.strictEqual(err.code, 'ERR_ASSERTION');
      assert.strictEqual(err.operator, 'strictEqual');
      assert.strictEqual(err.generatedMessage, true);
    }
    javascript

### Class: `assert.Assert`#

Added in: v24.6.0, v22.19.0

The `Assert` class allows creating independent assertion instances with custom options.

#### `new assert.Assert([options])`#

History

Version | Changes
--- | ---
v24.9.0 | Added `skipPrototype` option.

- `options` `<Object>`
  - `diff` `<string>` If set to `'full'`, shows the full diff in assertion errors. Defaults to `'simple'`. Accepted values: `'simple'`, `'full'`.
  - `strict` `<boolean>` If set to `true`, non-strict methods behave like their corresponding strict methods. Defaults to `true`.
  - `skipPrototype` `<boolean>` If set to `true`, skips prototype and constructor comparison in deep equality checks. Defaults to `false`.

Creates a new assertion instance. The `diff` option controls the verbosity of diffs in assertion error messages.

    const { Assert } = require('node:assert');
    const assertInstance = new Assert({ diff: 'full' });
    assertInstance.deepStrictEqual({ a: 1 }, { a: 2 });
    // Shows a full diff in the error message.
    js

Important: When destructuring assertion methods from an `Assert` instance, the methods lose their connection to the instance's configuration options (such as `diff`, `strict`, and `skipPrototype` settings). The destructured methods will fall back to default behavior instead.

    const myAssert = new Assert({ diff: 'full' });

    // This works as expected - uses 'full' diff
    myAssert.strictEqual({ a: 1 }, { b: { c: 1 } });

    // This loses the 'full' diff setting - falls back to default 'simple' diff
    const { strictEqual } = myAssert;
    strictEqual({ a: 1 }, { b: { c: 1 } });
    js

The `skipPrototype` option affects all deep equality methods:

    class Foo {
      constructor(a) {
        this.a = a;
      }
    }

    class Bar {
      constructor(a) {
        this.a = a;
      }
    }

    const foo = new Foo(1);
    const bar = new Bar(1);

    // Default behavior - fails due to different constructors
    const assert1 = new Assert();
    assert1.deepStrictEqual(foo, bar); // AssertionError

    // Skip prototype comparison - passes if properties are equal
    const assert2 = new Assert({ skipPrototype: true });
    assert2.deepStrictEqual(foo, bar); // OK
    js

When destructured, methods lose access to the instance's `this` context and revert to default assertion behavior (diff: 'simple', non-strict mode). To maintain custom options when using destructured methods, avoid destructuring and call methods directly on the instance.

### `assert(value[, message])`#

Added in: v0.5.9
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.

- `value` `<any>` The input that is checked for being truthy.
- `message` `<string>` | `<Error>` | `<Function>`

An alias of `assert.ok()`.

### `assert.deepEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0 | Recursion now stops when either side encounters a circular reference.
v22.2.0, v20.15.0 | Error cause and errors properties are now compared as well.
v18.0.0 | Regular expressions lastIndex property is now compared as well.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.
v12.0.0 | The type tags are now properly compared and there are a couple minor comparison adjustments to make the check less surprising.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.deepStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.deepStrictEqual()` instead.

Tests for deep equality between the `actual` and `expected` parameters. Consider using `assert.deepStrictEqual()` instead. `assert.deepEqual()` can have surprising results.

Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.

#### Comparison details#

- Primitive values are compared with the `==` operator, with the exception of `<NaN>`. It is treated as being identical in case both sides are `<NaN>`.
- Type tags of objects should be the same.
- Only enumerable "own" properties are considered.
- Object constructors are compared when available.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or either side encounters a circular reference.
- Implementation does not test the `[[Prototype]]` of objects.
- `<Symbol>` properties are not compared.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.

The following example does not throw an `AssertionError` because the primitives are compared using the `==` operator.

    import assert from 'node:assert';
    // WARNING: This does not throw an AssertionError!

    assert.deepEqual('+00000000', false);
    const assert = require('node:assert');
    // WARNING: This does not throw an AssertionError!

    assert.deepEqual('+00000000', false);
    javascript

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

    import assert from 'node:assert';

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.deepEqual(obj1, obj1);
    // OK

    // Values of b are different:
    assert.deepEqual(obj1, obj2);
    // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

    assert.deepEqual(obj1, obj3);
    // OK

    // Prototypes are ignored:
    assert.deepEqual(obj1, obj4);
    // AssertionError: { a: { b: 1 } } deepEqual {}
    const assert = require('node:assert');

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.deepEqual(obj1, obj1);
    // OK

    // Values of b are different:
    assert.deepEqual(obj1, obj2);
    // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

    assert.deepEqual(obj1, obj3);
    // OK

    // Prototypes are ignored:
    assert.deepEqual(obj1, obj4);
    // AssertionError: { a: { b: 1 } } deepEqual {}
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.deepStrictEqual(actual, expected[, message])`#

Added in: v1.2.0
History

Version | Changes
--- | ---
v25.1.0 | Message may now be a `printf`-like format string or function.
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0 | Recursion now stops when either side encounters a circular reference.
v22.2.0, v20.15.0 | Error cause and errors properties are now compared as well.
v18.0.0 | Regular expressions lastIndex property is now compared as well.
v9.0.0 | Enumerable symbol properties are now compared.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v8.5.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for deep equality between the `actual` and `expected` parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules.

#### Comparison details#

- Primitive values are compared using `Object.is()`.
- Type tags of objects should be the same.
- `[[Prototype]]` of objects are compared using the `===` operator.
- Only enumerable "own" properties are considered.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties. `errors` is also compared.
- Enumerable own `<Symbol>` properties are compared as well.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or either side encounters a circular reference.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.

    import assert from 'node:assert/strict';

    // This fails because 1 !== '1'.
    assert.deepStrictEqual({ a: 1 }, { a: '1' });
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    //   {
    // +   a: 1
    // -   a: '1'
    //   }

    // The following objects don't have own properties
    const date = new Date();
    const object = {};
    const fakeDate = {};
    Object.setPrototypeOf(fakeDate, Date.prototype);

    // Different [[Prototype]]:
    assert.deepStrictEqual(object, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + {}
    // - Date {}

    // Different type tags:
    assert.deepStrictEqual(date, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 2018-04-26T00:49:08.604Z
    // - Date {}

    assert.deepStrictEqual(NaN, NaN);
    // OK because Object.is(NaN, NaN) is true.

    // Different unwrapped numbers:
    assert.deepStrictEqual(new Number(1), new Number(2));
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + [Number: 1]
    // - [Number: 2]

    assert.deepStrictEqual(new String('foo'), Object('foo'));
    // OK because the object and the string are identical when unwrapped.

    assert.deepStrictEqual(-0, -0);
    // OK

    // Different zeros:
    assert.deepStrictEqual(0, -0);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 0
    // - -0

    const symbol1 = Symbol();
    const symbol2 = Symbol();
    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
    // OK, because it is the same symbol on both objects.

    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
    // AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
    //
    // {
    //   Symbol(): 1
    // }

    const weakMap1 = new WeakMap();
    const weakMap2 = new WeakMap();
    const obj = {};

    weakMap1.set(obj, 'value');
    weakMap2.set(obj, 'value');

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakMap1, weakMap2);
    // AssertionError: Values have same structure but are not reference-equal:
    //
    // WeakMap {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakMap1, weakMap1);
    // OK

    const weakSet1 = new WeakSet();
    const weakSet2 = new WeakSet();
    weakSet1.add(obj);
    weakSet2.add(obj);

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakSet1, weakSet2);
    // AssertionError: Values have same structure but are not reference-equal:
    // + actual - expected
    //
    // WeakSet {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakSet1, weakSet1);
    // OK
    const assert = require('node:assert/strict');

    // This fails because 1 !== '1'.
    assert.deepStrictEqual({ a: 1 }, { a: '1' });
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    //   {
    // +   a: 1
    // -   a: '1'
    //   }

    // The following objects don't have own properties
    const date = new Date();
    const object = {};
    const fakeDate = {};
    Object.setPrototypeOf(fakeDate, Date.prototype);

    // Different [[Prototype]]:
    assert.deepStrictEqual(object, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + {}
    // - Date {}

    // Different type tags:
    assert.deepStrictEqual(date, fakeDate);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 2018-04-26T00:49:08.604Z
    // - Date {}

    assert.deepStrictEqual(NaN, NaN);
    // OK because Object.is(NaN, NaN) is true.

    // Different unwrapped numbers:
    assert.deepStrictEqual(new Number(1), new Number(2));
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + [Number: 1]
    // - [Number: 2]

    assert.deepStrictEqual(new String('foo'), Object('foo'));
    // OK because the object and the string are identical when unwrapped.

    assert.deepStrictEqual(-0, -0);
    // OK

    // Different zeros:
    assert.deepStrictEqual(0, -0);
    // AssertionError: Expected inputs to be strictly deep-equal:
    // + actual - expected
    //
    // + 0
    // - -0

    const symbol1 = Symbol();
    const symbol2 = Symbol();
    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
    // OK, because it is the same symbol on both objects.

    assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
    // AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
    //
    // {
    //   Symbol(): 1
    // }

    const weakMap1 = new WeakMap();
    const weakMap2 = new WeakMap();
    const obj = {};

    weakMap1.set(obj, 'value');
    weakMap2.set(obj, 'value');

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakMap1, weakMap2);
    // AssertionError: Values have same structure but are not reference-equal:
    //
    // WeakMap {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakMap1, weakMap1);
    // OK

    const weakSet1 = new WeakSet();
    const weakSet2 = new WeakSet();
    weakSet1.add(obj);
    weakSet2.add(obj);

    // Comparing different instances fails, even with same contents
    assert.deepStrictEqual(weakSet1, weakSet2);
    // AssertionError: Values have same structure but are not reference-equal:
    // + actual - expected
    //
    // WeakSet {
    //   <items unknown>
    // }

    // Comparing the same instance to itself succeeds
    assert.deepStrictEqual(weakSet1, weakSet1);
    // OK
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.doesNotMatch(string, regexp[, message])`#

Added in: v13.6.0, v12.16.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0 | This API is no longer experimental.

- `string` `<string>`
- `regexp` `<RegExp>`
- `message` `<string>` | `<Error>` | `<Function>`

Expects the `string` input not to match the regular expression.

    import assert from 'node:assert/strict';

    assert.doesNotMatch('I will fail', /fail/);
    // AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

    assert.doesNotMatch(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.doesNotMatch('I will pass', /different/);
    // OK
    const assert = require('node:assert/strict');

    assert.doesNotMatch('I will fail', /fail/);
    // AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

    assert.doesNotMatch(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.doesNotMatch('I will pass', /different/);
    // OK
    javascript

If the values do match, or if the `string` argument is of another type than `string`, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.doesNotReject(asyncFn[, error][, message])`#

Added in: v10.0.0

- `asyncFn` `<Function>` | `<Promise>`
- `error` `<RegExp>` | `<Function>`
- `message` `<string>`
- Returns: `<Promise>`

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected.

If `asyncFn` is a function and it throws an error synchronously, `assert.doesNotReject()` will return a rejected `Promise` with that error. If the function does not return a promise, `assert.doesNotReject()` will return a rejected `Promise` with an `ERR_INVALID_RETURN_VALUE` error. In both cases the error handler is skipped.

Using `assert.doesNotReject()` is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible.

If specified, `error` can be a `Class`, `<RegExp>` or a validation function. See `assert.throws()` for more details.

Besides the async nature to await the completion behaves identically to `assert.doesNotThrow()`.

    import assert from 'node:assert/strict';

    await assert.doesNotReject(
      async () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.doesNotReject(
        async () => {
          throw new TypeError('Wrong value');
        },
        SyntaxError,
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });
    const assert = require('node:assert/strict');

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });
    javascript

### `assert.doesNotThrow(fn[, error][, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v5.11.0, v4.4.5 | The `message` parameter is respected now.
v4.2.0 | The `error` parameter can now be an arrow function.

- `fn` `<Function>`
- `error` `<RegExp>` | `<Function>`
- `message` `<string>`

Asserts that the function `fn` does not throw an error.

Using `assert.doesNotThrow()` is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible.

When `assert.doesNotThrow()` is called, it will immediately call the `fn` function.

If an error is thrown and it is the same type as that specified by the `error` parameter, then an `AssertionError` is thrown. If the error is of a different type, or if the `error` parameter is undefined, the error is propagated back to the caller.

If specified, `error` can be a `Class`, `<RegExp>`, or a validation function. See `assert.throws()` for more details.

The following, for instance, will throw the `<TypeError>` because there is no matching error type in the assertion:

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError,
    );
    javascript

However, the following will result in an `AssertionError` with the message 'Got unwanted exception...':

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      TypeError,
    );
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      TypeError,
    );
    javascript

If an `AssertionError` is thrown and a value is provided for the `message` parameter, the value of `message` will be appended to the `AssertionError` message:

    import assert from 'node:assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops',
    );
    // Throws: AssertionError: Got unwanted exception: Whoops
    const assert = require('node:assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops',
    );
    // Throws: AssertionError: Got unwanted exception: Whoops
    javascript

### `assert.equal(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.strictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.strictEqual()` instead.

Tests shallow, coercive equality between the `actual` and `expected` parameters using the `==` operator. `NaN` is specially handled and treated as being identical if both sides are `NaN`.

    import assert from 'node:assert';

    assert.equal(1, 1);
    // OK, 1 == 1
    assert.equal(1, '1');
    // OK, 1 == '1'
    assert.equal(NaN, NaN);
    // OK

    assert.equal(1, 2);
    // AssertionError: 1 == 2
    assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
    // AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
    const assert = require('node:assert');

    assert.equal(1, 1);
    // OK, 1 == 1
    assert.equal(1, '1');
    // OK, 1 == '1'
    assert.equal(NaN, NaN);
    // OK

    assert.equal(1, 2);
    // AssertionError: 1 == 2
    assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
    // AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
    javascript

If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.fail([message])`#

Added in: v0.1.21

- `message` `<string>` | `<Error>` Default: `'Failed'`

Throws an `AssertionError` with the provided error message or a default error message. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

    import assert from 'node:assert/strict';

    assert.fail();
    // AssertionError [ERR_ASSERTION]: Failed

    assert.fail('boom');
    // AssertionError [ERR_ASSERTION]: boom

    assert.fail(new TypeError('need array'));
    // TypeError: need array
    const assert = require('node:assert/strict');

    assert.fail();
    // AssertionError [ERR_ASSERTION]: Failed

    assert.fail('boom');
    // AssertionError [ERR_ASSERTION]: boom

    assert.fail(new TypeError('need array'));
    // TypeError: need array
    javascript

### `assert.ifError(value)`#

Added in: v0.1.97
History

Version | Changes
--- | ---
v10.0.0 | Instead of throwing the original error it is now wrapped into an [`AssertionError`][] that contains the full stack trace.
v10.0.0 | Value may now only be `undefined` or `null`. Before all falsy values were handled the same as `null` and did not throw.

- `value` `<any>`

Throws `value` if `value` is not `undefined` or `null`. This is useful when testing the `error` argument in callbacks. The stack trace contains all frames from the error passed to `ifError()` including the potential new frames for `ifError()` itself.

    import assert from 'node:assert/strict';

    assert.ifError(null);
    // OK
    assert.ifError(0);
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
    assert.ifError('error');
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
    assert.ifError(new Error());
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

    // Create some random error frames.
    let err;
    (function errorFrame() {
      err = new Error('test error');
    })();

    (function ifErrorFrame() {
      assert.ifError(err);
    })();
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
    //     at ifErrorFrame
    //     at errorFrame
    const assert = require('node:assert/strict');

    assert.ifError(null);
    // OK
    assert.ifError(0);
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
    assert.ifError('error');
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
    assert.ifError(new Error());
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

    // Create some random error frames.
    let err;
    (function errorFrame() {
      err = new Error('test error');
    })();

    (function ifErrorFrame() {
      assert.ifError(err);
    })();
    // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
    //     at ifErrorFrame
    //     at errorFrame
    javascript

### `assert.match(string, regexp[, message])`#

Added in: v13.6.0, v12.16.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0 | This API is no longer experimental.

- `string` `<string>`
- `regexp` `<RegExp>`
- `message` `<string>` | `<Error>` | `<Function>`

Expects the `string` input to match the regular expression.

    import assert from 'node:assert/strict';

    assert.match('I will fail', /pass/);
    // AssertionError [ERR_ASSERTION]: The input did not match the regular ...

    assert.match(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.match('I will pass', /pass/);
    // OK
    const assert = require('node:assert/strict');

    assert.match('I will fail', /pass/);
    // AssertionError [ERR_ASSERTION]: The input did not match the regular ...

    assert.match(123, /pass/);
    // AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

    assert.match('I will pass', /pass/);
    // OK
    javascript

If the values do not match, or if the `string` argument is of another type than `string`, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notDeepEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.notDeepStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.notDeepStrictEqual()` instead.

Tests for any deep inequality. Opposite of `assert.deepEqual()`.

    import assert from 'node:assert';

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.notDeepEqual(obj1, obj1);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj2);
    // OK

    assert.notDeepEqual(obj1, obj3);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj4);
    // OK
    const assert = require('node:assert');

    const obj1 = {
      a: {
        b: 1,
      },
    };
    const obj2 = {
      a: {
        b: 2,
      },
    };
    const obj3 = {
      a: {
        b: 1,
      },
    };
    const obj4 = { __proto__: obj1 };

    assert.notDeepEqual(obj1, obj1);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj2);
    // OK

    assert.notDeepEqual(obj1, obj3);
    // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

    assert.notDeepEqual(obj1, obj4);
    // OK
    javascript

If the values are deeply equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notDeepStrictEqual(actual, expected[, message])`#

Added in: v1.2.0
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v9.0.0 | The `-0` and `+0` are not considered equal anymore.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for deep strict inequality. Opposite of `assert.deepStrictEqual()`.

    import assert from 'node:assert/strict';

    assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
    // OK
    const assert = require('node:assert/strict');

    assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
    // OK
    javascript

If the values are deeply and strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v16.0.0, v14.18.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical if both sides are NaN.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Strict assertion mode

An alias of `assert.notStrictEqual()`.

Legacy assertion mode

Stability: 3 - Legacy: Use `assert.notStrictEqual()` instead.

Tests shallow, coercive inequality with the `!=` operator. `NaN` is specially handled and treated as being identical if both sides are `NaN`.

    import assert from 'node:assert';

    assert.notEqual(1, 2);
    // OK

    assert.notEqual(1, 1);
    // AssertionError: 1 != 1

    assert.notEqual(1, '1');
    // AssertionError: 1 != '1'
    const assert = require('node:assert');

    assert.notEqual(1, 2);
    // OK

    assert.notEqual(1, 1);
    // AssertionError: 1 != 1

    assert.notEqual(1, '1');
    // AssertionError: 1 != '1'
    javascript

If the values are equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.notStrictEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests strict inequality between the `actual` and `expected` parameters as determined by `Object.is()`.

    import assert from 'node:assert/strict';

    assert.notStrictEqual(1, 2);
    // OK

    assert.notStrictEqual(1, 1);
    // AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
    //
    // 1

    assert.notStrictEqual(1, '1');
    // OK
    const assert = require('node:assert/strict');

    assert.notStrictEqual(1, 2);
    // OK

    assert.notStrictEqual(1, 1);
    // AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
    //
    // 1

    assert.notStrictEqual(1, '1');
    // OK
    javascript

If the values are strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.ok(value[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | The `assert.ok()` (no arguments) will now use a predefined error message.

- `value` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests if `value` is truthy. It is equivalent to `assert.equal(!!value, true, message)`.

If `value` is not truthy, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is `undefined`, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`. If no arguments are passed in at all `message` will be set to the string: `'No value argument passed to `assert.ok()`'`.

Be aware that in the `repl` the error message will be different to the one thrown in a file! See below for further details.

    import assert from 'node:assert/strict';

    assert.ok(true);
    // OK
    assert.ok(1);
    // OK

    assert.ok();
    // AssertionError: No value argument passed to `assert.ok()`

    assert.ok(false, 'it\'s false');
    // AssertionError: it's false

    // In the repl:
    assert.ok(typeof 123 === 'string');
    // AssertionError: false == true

    // In a file (e.g. test.js):
    assert.ok(typeof 123 === 'string');
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(typeof 123 === 'string')

    assert.ok(false);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(false)

    assert.ok(0);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(0)
    const assert = require('node:assert/strict');

    assert.ok(true);
    // OK
    assert.ok(1);
    // OK

    assert.ok();
    // AssertionError: No value argument passed to `assert.ok()`

    assert.ok(false, 'it\'s false');
    // AssertionError: it's false

    // In the repl:
    assert.ok(typeof 123 === 'string');
    // AssertionError: false == true

    // In a file (e.g. test.js):
    assert.ok(typeof 123 === 'string');
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(typeof 123 === 'string')

    assert.ok(false);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(false)

    assert.ok(0);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert.ok(0)
    javascript

    import assert from 'node:assert/strict';

    // Using `assert()` works the same:
    assert(2 + 2 > 5);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert(2 + 2 > 5)
    const assert = require('node:assert');

    // Using `assert()` works the same:
    assert(2 + 2 > 5);
    // AssertionError: The expression evaluated to a falsy value:
    //
    //   assert(2 + 2 > 5)
    javascript

### `assert.rejects(asyncFn[, error][, message])`#

Added in: v10.0.0

- `asyncFn` `<Function>` | `<Promise>`
- `error` `<RegExp>` | `<Function>` | `<Object>` | `<Error>`
- `message` `<string>`
- Returns: `<Promise>`

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected.

If `asyncFn` is a function and it throws an error synchronously, `assert.rejects()` will return a rejected `Promise` with that error. If the function does not return a promise, `assert.rejects()` will return a rejected `Promise` with an `ERR_INVALID_RETURN_VALUE` error. In both cases the error handler is skipped.

Besides the async nature to await the completion behaves identically to `assert.throws()`.

If specified, `error` can be a `Class`, `<RegExp>`, a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable `message` and `name` properties.

If specified, `message` will be the message provided by the `AssertionError` if the `asyncFn` fails to reject.

    import assert from 'node:assert/strict';

    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
      },
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.rejects(
        async () => {
          throw new TypeError('Wrong value');
        },
        {
          name: 'TypeError',
          message: 'Wrong value',
        },
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      (err) => {
        assert.strictEqual(err.name, 'TypeError');
        assert.strictEqual(err.message, 'Wrong value');
        return true;
      },
    );
    const assert = require('node:assert/strict');

    (async () => {
      await assert.rejects(
        async () => {
          throw new TypeError('Wrong value');
        },
        (err) => {
          assert.strictEqual(err.name, 'TypeError');
          assert.strictEqual(err.message, 'Wrong value');
          return true;
        },
      );
    })();
    javascript

    import assert from 'node:assert/strict';

    assert.rejects(
      Promise.reject(new Error('Wrong value')),
      Error,
    ).then(() => {
      // ...
    });
    const assert = require('node:assert/strict');

    assert.rejects(
      Promise.reject(new Error('Wrong value')),
      Error,
    ).then(() => {
      // ...
    });
    javascript

`error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Please read the example in `assert.throws()` carefully if using a string as the second argument gets considered.

### `assert.strictEqual(actual, expected[, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v26.0.0 | Message may now be a `printf`-like format string or function.
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>` Postfix `printf`-like arguments in case it's used as format string. If message is a function, it is called in case of a comparison failure. The function receives the `actual` and `expected` arguments and has to return a string that is going to be used as error message. `printf`-like format strings and functions are beneficial for performance reasons in case arguments are passed through. In addition, it allows nice formatting with ease.

Tests strict equality between the `actual` and `expected` parameters as determined by `Object.is()`.

    import assert from 'node:assert/strict';

    assert.strictEqual(1, 2);
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    //
    // 1 !== 2

    assert.strictEqual(1, 1);
    // OK

    assert.strictEqual('Hello foobar', 'Hello World!');
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    // + actual - expected
    //
    // + 'Hello foobar'
    // - 'Hello World!'
    //          ^

    const apples = 1;
    const oranges = 2;
    assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
    // TypeError: Inputs are not identical

    assert.strictEqual(apples, oranges, (actual, expected) => {
      // Do 'heavy' computations
      return `I expected ${expected} but I got ${actual}`;
    });
    // AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
    const assert = require('node:assert/strict');

    assert.strictEqual(1, 2);
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    //
    // 1 !== 2

    assert.strictEqual(1, 1);
    // OK

    assert.strictEqual('Hello foobar', 'Hello World!');
    // AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
    // + actual - expected
    //
    // + 'Hello foobar'
    // - 'Hello World!'
    //          ^

    const apples = 1;
    const oranges = 2;
    assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
    // AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

    assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
    // TypeError: Inputs are not identical

    assert.strictEqual(apples, oranges, (actual, expected) => {
      // Do 'heavy' computations
      return `I expected ${expected} but I got ${actual}`;
    });
    // AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
    javascript

If the values are not strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of `<Error>` then it will be thrown instead of the `AssertionError`.

### `assert.throws(fn[, error][, message])`#

Added in: v0.1.21
History

Version | Changes
--- | ---
v10.2.0 | The `error` parameter can be an object containing regular expressions now.
v9.9.0 | The `error` parameter can now be an object as well.
v4.2.0 | The `error` parameter can now be an arrow function.

- `fn` `<Function>`
- `error` `<RegExp>` | `<Function>` | `<Object>` | `<Error>`
- `message` `<string>`

Expects the function `fn` to throw an error.

If specified, `error` can be a `Class`, `<RegExp>`, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable `message` and `name` properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

If specified, `message` will be appended to the message provided by the `AssertionError` if the `fn` call fails to throw or in case the error validation fails.

Custom validation object/error instance:

    import assert from 'node:assert/strict';

    const err = new TypeError('Wrong value');
    err.code = 404;
    err.foo = 'bar';
    err.info = {
      nested: true,
      baz: 'text',
    };
    err.reg = /abc/i;

    assert.throws(
      () => {
        throw err;
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
        info: {
          nested: true,
          baz: 'text',
        },
        // Only properties on the validation object will be tested for.
        // Using nested objects requires all properties to be present. Otherwise
        // the validation is going to fail.
      },
    );

    // Using regular expressions to validate error properties:
    assert.throws(
      () => {
        throw err;
      },
      {
        // The `name` and `message` properties are strings and using regular
        // expressions on those will match against the string. If they fail, an
        // error is thrown.
        name: /^TypeError$/,
        message: /Wrong/,
        foo: 'bar',
        info: {
          nested: true,
          // It is not possible to use regular expressions for nested properties!
          baz: 'text',
        },
        // The `reg` property contains a regular expression and only if the
        // validation object contains an identical regular expression, it is going
        // to pass.
        reg: /abc/i,
      },
    );

    // Fails due to the different `message` and `name` properties:
    assert.throws(
      () => {
        const otherErr = new Error('Not found');
        // Copy all enumerable properties from `err` to `otherErr`.
        for (const [key, value] of Object.entries(err)) {
          otherErr[key] = value;
        }
        throw otherErr;
      },
      // The error's `message` and `name` properties will also be checked when using
      // an error as validation object.
      err,
    );
    const assert = require('node:assert/strict');

    const err = new TypeError('Wrong value');
    err.code = 404;
    err.foo = 'bar';
    err.info = {
      nested: true,
      baz: 'text',
    };
    err.reg = /abc/i;

    assert.throws(
      () => {
        throw err;
      },
      {
        name: 'TypeError',
        message: 'Wrong value',
        info: {
          nested: true,
          baz: 'text',
        },
        // Only properties on the validation object will be tested for.
        // Using nested objects requires all properties to be present. Otherwise
        // the validation is going to fail.
      },
    );

    // Using regular expressions to validate error properties:
    assert.throws(
      () => {
        throw err;
      },
      {
        // The `name` and `message` properties are strings and using regular
        // expressions on those will match against the string. If they fail, an
        // error is thrown.
        name: /^TypeError$/,
        message: /Wrong/,
        foo: 'bar',
        info: {
          nested: true,
          // It is not possible to use regular expressions for nested properties!
          baz: 'text',
        },
        // The `reg` property contains a regular expression and only if the
        // validation object contains an identical regular expression, it is going
        // to pass.
        reg: /abc/i,
      },
    );

    // Fails due to the different `message` and `name` properties:
    assert.throws(
      () => {
        const otherErr = new Error('Not found');
        // Copy all enumerable properties from `err` to `otherErr`.
        for (const [key, value] of Object.entries(err)) {
          otherErr[key] = value;
        }
        throw otherErr;
      },
      // The error's `message` and `name` properties will also be checked when using
      // an error as validation object.
      err,
    );
    javascript

Validate instanceof using constructor:

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      Error,
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      Error,
    );
    javascript

Validate error message using `<RegExp>`:

Using a regular expression runs `.toString` on the error object, and will therefore also include the error name.

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      /^Error: Wrong value$/,
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      /^Error: Wrong value$/,
    );
    javascript

Custom error validation:

The function must return `true` to indicate all internal validations passed. It will otherwise fail with an `AssertionError`.

    import assert from 'node:assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      (err) => {
        assert(err instanceof Error);
        assert(/value/.test(err));
        // Avoid returning anything from validation functions besides `true`.
        // Otherwise, it's not clear what part of the validation failed. Instead,
        // throw an error about the specific validation that failed (as done in this
        // example) and add as much helpful debugging information to that error as
        // possible.
        return true;
      },
      'unexpected error',
    );
    const assert = require('node:assert/strict');

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      (err) => {
        assert(err instanceof Error);
        assert(/value/.test(err));
        // Avoid returning anything from validation functions besides `true`.
        // Otherwise, it's not clear what part of the validation failed. Instead,
        // throw an error about the specific validation that failed (as done in this
        // example) and add as much helpful debugging information to that error as
        // possible.
        return true;
      },
      'unexpected error',
    );
    javascript

`error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an `ERR_AMBIGUOUS_ARGUMENT` error. Please read the example below carefully if using a string as the second argument gets considered:

    import assert from 'node:assert/strict';

    function throwingFirst() {
      throw new Error('First');
    }

    function throwingSecond() {
      throw new Error('Second');
    }

    function notThrowing() {}

    // The second argument is a string and the input function threw an Error.
    // The first case will not throw as it does not match for the error message
    // thrown by the input function!
    assert.throws(throwingFirst, 'Second');
    // In the next example the message has no benefit over the message from the
    // error and since it is not clear if the user intended to actually match
    // against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
    assert.throws(throwingSecond, 'Second');
    // TypeError [ERR_AMBIGUOUS_ARGUMENT]

    // The string is only used (as message) in case the function does not throw:
    assert.throws(notThrowing, 'Second');
    // AssertionError [ERR_ASSERTION]: Missing expected exception: Second

    // If it was intended to match for the error message do this instead:
    // It does not throw because the error messages match.
    assert.throws(throwingSecond, /Second$/);

    // If the error message does not match, an AssertionError is thrown.
    assert.throws(throwingFirst, /Second$/);
    // AssertionError [ERR_ASSERTION]
    const assert = require('node:assert/strict');

    function throwingFirst() {
      throw new Error('First');
    }

    function throwingSecond() {
      throw new Error('Second');
    }

    function notThrowing() {}

    // The second argument is a string and the input function threw an Error.
    // The first case will not throw as it does not match for the error message
    // thrown by the input function!
    assert.throws(throwingFirst, 'Second');
    // In the next example the message has no benefit over the message from the
    // error and since it is not clear if the user intended to actually match
    // against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
    assert.throws(throwingSecond, 'Second');
    // TypeError [ERR_AMBIGUOUS_ARGUMENT]

    // The string is only used (as message) in case the function does not throw:
    assert.throws(notThrowing, 'Second');
    // AssertionError [ERR_ASSERTION]: Missing expected exception: Second

    // If it was intended to match for the error message do this instead:
    // It does not throw because the error messages match.
    assert.throws(throwingSecond, /Second$/);

    // If the error message does not match, an AssertionError is thrown.
    assert.throws(throwingFirst, /Second$/);
    // AssertionError [ERR_ASSERTION]
    javascript

Due to the confusing error-prone notation, avoid a string as the second argument.

### `assert.partialDeepStrictEqual(actual, expected[, message])`#

Added in: v23.4.0, v22.13.0
History

Version | Changes
--- | ---
v25.0.0 | Promises are not considered equal anymore if they are not of the same instance.
v25.0.0 | Invalid dates are now considered equal.
v24.0.0, v22.17.0 | partialDeepStrictEqual is now Stable. Previously, it had been Experimental.

- `actual` `<any>`
- `expected` `<any>`
- `message` `<string>` | `<Error>` | `<Function>`

Tests for partial deep equality between the `actual` and `expected` parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules. "Partial" equality means that only properties that exist on the `expected` parameter are going to be compared.

This method always passes the same test cases as `assert.deepStrictEqual()`, behaving as a super set of it.

#### Comparison details#

- Primitive values are compared using `Object.is()`.
- Type tags of objects should be the same.
- `[[Prototype]]` of objects are not compared.
- Only enumerable "own" properties are considered.
- `<Error>` names, messages, causes, and errors are always compared, even if these are not enumerable properties. `errors` is also compared.
- Enumerable own `<Symbol>` properties are compared as well.
- Object wrappers are compared both as objects and unwrapped values.
- `Object` properties are compared unordered.
- `<Map>` keys and `<Set>` items are compared unordered.
- Recursion stops when both sides differ or both sides encounter a circular reference.
- `<WeakMap>`, `<WeakSet>` and `<Promise>` instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different `WeakMap`, `WeakSet`, or `Promise` instances will result in inequality, even if they contain the same content.
- `<RegExp>` lastIndex, flags, and source are always compared, even if these are not enumerable properties.
- Holes in sparse arrays are ignored.

    import assert from 'node:assert';

    assert.partialDeepStrictEqual(
      { a: { b: { c: 1 } } },
      { a: { b: { c: 1 } } },
    );
    // OK

    assert.partialDeepStrictEqual(
      { a: 1, b: 2, c: 3 },
      { b: 2 },
    );
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [4, 5, 8],
    );
    // OK

    assert.partialDeepStrictEqual(
      new Set([{ a: 1 }, { b: 1 }]),
      new Set([{ a: 1 }]),
    );
    // OK

    assert.partialDeepStrictEqual(
      new Map([['key1', 'value1'], ['key2', 'value2']]),
      new Map([['key2', 'value2']]),
    );
    // OK

    assert.partialDeepStrictEqual(123n, 123n);
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [5, 4, 8],
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: 1 },
      { a: 1, b: 2 },
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: { b: 2 } },
      { a: { b: '2' } },
    );
    // AssertionError
    const assert = require('node:assert');

    assert.partialDeepStrictEqual(
      { a: { b: { c: 1 } } },
      { a: { b: { c: 1 } } },
    );
    // OK

    assert.partialDeepStrictEqual(
      { a: 1, b: 2, c: 3 },
      { b: 2 },
    );
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [4, 5, 8],
    );
    // OK

    assert.partialDeepStrictEqual(
      new Set([{ a: 1 }, { b: 1 }]),
      new Set([{ a: 1 }]),
    );
    // OK

    assert.partialDeepStrictEqual(
      new Map([['key1', 'value1'], ['key2', 'value2']]),
      new Map([['key2', 'value2']]),
    );
    // OK

    assert.partialDeepStrictEqual(123n, 123n);
    // OK

    assert.partialDeepStrictEqual(
      [1, 2, 3, 4, 5, 6, 7, 8, 9],
      [5, 4, 8],
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: 1 },
      { a: 1, b: 2 },
    );
    // AssertionError

    assert.partialDeepStrictEqual(
      { a: { b: 2 } },
      { a: { b: '2' } },
    );
    // AssertionError
    javascript